1. Field of the Invention
The present invention relates in general to a gear structure and a method of producing the same, and more particularly, to a gear structure which has forged teeth formed thereon and a method of producing the same.
2. Description of the Prior Art
In order to clarify the task of the present invention, a seat reclining device to which one conventional gear structure is practically applied will be described with reference to FIG. 6 of the accompanying drawings.
The seat reclining device 1 comprises a base member 2 which is secured to a seat cushion part (not shown). A pivot shaft 3 is secured to the base member 2, which passes through an opening 5b of a toothed upper member 5. The toothed upper member 5 is secured to a pivot arm 4 which is secured to a lower portion of a seatback part (not shown). Thus, the seatback part is pivotal about the pivot shaft 3 relative to the seat cushion part. The teeth of the upper member is designated by numeral 5a. Latchingly engageable with the teeth 5a of the upper member 5 is a toothed lower member 7 which is pivotally supported by the base 2 through a pivot pin 6. Designated by numeral 7a is the teeth of the lower member, and 7b is an opening through which the pivot pin 6 passes. Although not shown in the drawing, suitable biasing means, such as spring or the like, is incorporated with the toothed lower member 7 to bias the same toward the toothed upper member 5. Thus, usually, the seatback part is kept locked at an desired angular position relative to the seat cushion part. However, when the toothed lower member 7 is disengaged from the toothed upper member 5 against the force of the biasing means, the seatback part is permitted to pivot freely relative to the seat cushion part.
It is known that, upon a seat occupant having his back pressed against the seatback part, a remarkable load is applied to the lockingly engaged teeth 5a and 7a of the upper and lower members 5 and 7. Thus, hitherto, in order to withstand such remarkable load, the upper and lower members 5 and 7 have been produced from a relatively thick metal plate by using, for example, so-called "hot gear rolling method".
In the hot gear rolling method, a toothed die is used, which is rotated to continuously press the toothed work portion thereof against a peripheral portion of a base metal plate where teeth are to be formed.
However, due to the inherency of this method, it is difficult to provide each tooth of the produced gear with a bilateral symmetry. That is, as is shown in FIG. 7, the method tends to fail to equalize the angles A and B of each tooth 10, each angle A (or B) being defined between a center plane 100 which passes through both a center (not shown) of the gear and a top of the tooth 10 and a side plane 100A (or 100B) which includes a front (or rear) side of the tooth 10. Furthermore, using a thicker metal plate causes not only increase in production cost of the toothed members but also weighty construction of the reclining device assembly.